已知数列{an}的前n项和为Sn,且Sn=n2(Ⅰ)求数列{an}的通项公式;(Ⅱ)记数列{1anan+1}的前n项和为Tn,

2025-01-05 14:25:31
推荐回答(1个)
回答1:

(I)当n=1时,a1=S1=1;当n≥2时,an=Sn-Sn-1=n2-(n-1)2=2n-1,
当n=1时适合上式,∴an=2n-1.(n∈N*).
(II)∵

1
anan+1
=
1
(2n?1)(2n+1)
=
1
2
(
1
2n?1
?
1
2n+1
)

∴数列{
1
anan+1
}的前n项和为Tn=
1
2
[(1?
1
3
)+(
1
3
?
1
5
)
+…+(
1
2n?1
?
1
2n+1
)]

=
1
2
(1?
1
2n+1
)

∵任意n∈N*,Tn
1
2
,对任意的n∈N*,Tn<m恒成立,
m≥
1
2

∴实数m的取值范围是[
1
2
,+∞)